Influence of spatially random soil on lateral thrust and failure surface in earth retaining walls

The present study investigates the influence of spatial variability of soil properties on the lateral thrust and failure surface of a 6?m high frictionless rigid earth retaining wall in active condition. The drained friction angle of the soil is modelled as a log-normal anisotropic random field in two dimensions, using the Cholesky decomposition technique. The effect of horizontal and vertical scale of fluctuation is observed on the lateral thrust. The failure surface in the backfill is found to be more or less similar for different combinations of spatial variability in vertical and horizontal directions. Monte-Carlo simulation technique is used to compute the probability of failure and to obtain the worst-case spatial variability configuration.     

Comment on: ‘High-temperature dehydration melting and decompressive P–T path in a granulite complex from the Eastern Ghats India’ by S. Bhattacharya and R. Kar

Thermodynamic and chemographic modelling of complex reaction textures observed in Mg-Al-rich pelitic granulites is an important tool to unravel the P–T evolutionary history of high-grade rocks. In the Eastern Ghats Belt, India, several studies have been carried out on these fascinating aluminous granulites, and the results of these studies have revealed complex P–T–t histories (Dasgupta and Sengupta 1995; Sengupta et al. 1999; Rickers et al. 2001a, 2001b; Gupta et al. 1999; Dobmeier and Simmat 2002; Dobmeier and Raith 2003). In recent communication, Bhattacharya and Kar (2002) reported reaction textures from a suite of Mg-Al granulites from the Paderu area of the Eastern Ghats Belt. Combining the textural relations and thermodynamic calibration of some construed reactions, the authors have put forward a single phase metamorphic evolution of the area along a clockwise pressure–temperature trajectory. Combining the petrological features from the Paderu area with those reported from the Chilka Lake complex, the authors proposed a general tectonic model for the entire Eastern Ghats Belt. Incidentally, the rocks in and around Paderu have been studied in some detail by several other workers (Lal et al. 1987; Mohan et al. 1997; Sengupta et al. 1997). The purpose of this comment is to demonstrate that the conclusions made in the paper are inconsistent with the petrological features described in the text. Further, the thermodynamic treatment used in the paper has serious errors in many places, and hence, is often in complete disagreement with the existing experimental data and theoretical analyses on the Mg-Al-rich assemblages. There are also significant problems arising from the poor quality of the analytical database. Unfortunately, the authors cite only a few published works (mostly their own) ignoring many other relevant studies from this belt (cited above). Our observations are organised according to the sections of the paper.Editorial responsibility: T.L. Grove     

A modified iterative sandwich method for determination of near-solidus partial melt compositions. II. Application to determination of near-solidus melt compositions of carbonated peridotite

We performed modified iterative sandwich experiments (MISE) to determine the composition of carbonatitic melt generated near the solidus of natural, fertile peridotite + CO₂ at 1,200–1,245°C and 6.6 GPa. Six iterations were performed with natural peridotite (MixKLB-1: Mg# = 89.7) and ∼10 wt% added carbonate to achieve the equilibrium carbonatite composition. Compositions of melts and coexisting minerals converged to a constant composition after the fourth iteration, with the silicate mineral compositions matching those expected at the solidus of carbonated peridotite at 6.6 GPa and 1,230°C, as determined from a sub-solidus experiment with MixKLB-1 peridotite. Partial melts expected from a carbonated lherzolite at a melt fraction of 0.01–0.05% at 6.6 GPa have the composition of sodic iron-bearing dolomitic carbonatite, with molar Ca/(Ca + Mg) of 0.413 ± 0.001, Ca# [100 × molar Ca/(Ca + Mg + Fe*)] of 37.1 ± 0.1, and Mg# of 83.7 ± 0.6. SiO₂, TiO₂ and Al₂O₃ concentrations are 4.1 ± 0.1, 1.0 ± 0.1, and 0.30 ± 0.02 wt%, whereas the Na₂O concentration is 4.0 ± 0.2 wt%. Comparison of our results with other iterative sandwich experiments at lower pressures indicate that near-solidus carbonatite derived from mantle lherzolite become less calcic with increasing pressure. Thus carbonatitic melt percolating through the deep mantle must dissolve cpx from surrounding peridotite and precipitate opx. Significant FeO* and Na₂O concentrations in near solidus carbonatitic partial melt likely account for the ∼150°C lower solidus temperature of natural carbonated peridotite compared to the solidus of synthetic peridotite in the system CMAS + CO₂. The experiments demonstrate that the MISE method can determine the composition of partial melts at very low melt fraction after a small number of iterations.     

Seismicity pattern in north Sumatra-Great Nicobar region: In search of precursor for the 26 December 2004 earthquake

We analyse the seismicity pattern including b-value in the north Sumatra-Great Nicobar region from 1976 to 2004. The analysis suggests that there were a number of significant, intermediate and short-term precursors before the magnitude 7.6 earthquake of 2 November 2002. However, they were not found to be so prominent prior to the magnitude 9.0 earthquake of 26 December 2004 though downward migration of activity and a 50-day short-term quiescence was observed before the event. The various precursors identified include post-seismic and intermediate-term quiescence of 13 and 10 years respectively, between the 1976 (magnitude 6.3) and 2002 earthquakes with two years (1990–1991) of increase in background seismicity; renewed seismicity, downward migration of seismic activity and foreshocks in 2002, just before the mainshock. Spatial variation in b-value with time indicates precursory changes in the form of high b-value zone near the epicenter preceding the mainshocks of 2004 and 2002 and temporal rise in b-value in the epicentral area before the 2002 earthquake.     

The effect of bulk composition on the solidus of carbonated eclogite from partial melting experiments at 3 GPa

To explore the effect of bulk composition on the solidus of carbonated eclogite, we determined near-solidus phase relations at 3 GPa for four different nominally anhydrous, carbonated eclogites. Starting materials (SLEC1, SLEC2, SLEC3, and SLEC4) were prepared by adding variable proportions and compositions of carbonate to a natural eclogite xenolith (66039B) from Salt Lake crater, Hawaii. Near-solidus partial melts for all bulk compositions are Fe–Na calcio-dolomitic and coexist with garnet + clinopyroxene + ilmenite ± calcio-dolomitic solid solution. The solidus for SLEC1 (Ca#=100 × molar Ca/(Ca + Mg + Fe_T)=32, 1.63 wt% Na₂O, and 5 wt% CO₂) is bracketed between 1,050°C and 1,075°C (Dasgupta et al. in Earth Planet Sci Lett 227:73–85, 2004), whereas initial melting for SLEC3 (Ca# 41, 1.4 wt% Na₂O, and 4.4 wt% CO₂) is between 1,175°C and 1,200°C. The solidus for SLEC2 (Ca# 33, 1.75 wt% Na₂O, and 15 wt% CO₂) is estimated to be near 1,100°C and the solidus for SLEC3 (Ca# 37, 1.47 wt% Na₂O, and 2.2 wt% CO₂) is between 1,100°C and 1,125°C. Solidus temperatures increase with increasing Ca# of the bulk, owing to the strong influence of the calcite–magnesite binary solidus-minimum on the solidus of carbonate bearing eclogite. Bulk compositions that produce near-solidus crystalline carbonate closer in composition to the minimum along the CaCO₃-MgCO₃ join have lower solidus temperatures. Variations in total CO₂ have significant effect on the solidus if CO₂ is added as CaCO₃, but not if CO₂ is added as a complex mixture that maintains the cationic ratios of the bulk-rock. Thus, as partial melting experiments necessarily have more CO₂ than that likely to be found in natural carbonated eclogites, care must be taken to assure that the compositional shifts associated with excess CO₂ do not unduly influence melting behavior. Near-solidus dolomite and calcite solid solutions have higher Ca/(Ca + Mg) than bulk eclogite compositions, owing to Ca–Mg exchange equilibrium between carbonates and silicates. Carbonates in natural mantle eclogite, which have low bulk CO₂ concentration, will have Ca/Mg buffered by reactions with silicates. Consequently, experiments with high bulk CO₂ may not mimic natural carbonated eclogite phase equilibria unless care is taken to ensure that CO₂ enrichment does not result in inappropriate equilibrium carbonate compositions. Compositions of eclogite-derived carbonate melt span the range of natural carbonatites from oceanic and continental settings. Ca#s of carbonatitic partial melts of eclogite vary significantly and overlap those of partial melts of carbonated lherzolite, however, for a constant Ca-content, Mg# of carbonatites derived from eclogitic sources are likely to be lower than the Mg# of those generated from peridotite.     

Relativistic and non-relativistic dust grains

Charged dust grains of radiia≃3×10⁻⁶∼3×10⁻⁵ cm could be a help in understanding high energy particles in extensive air showers (EAS). It is suggested that the dust grains in the intergalactic medium may attain relatistic energy (≥10²⁰ eV), and may be responsible for the apparent ‘bump’ in the energy spectrum. The relativistic and non-relativistic dust grains may help to understand the anomalies in the energy spectrum as regards the slope and intensity.     

Exact solution of transport equation in finite multiplying media

One speed neutron transport equation in a finite multiplying medium is exactly and uniquely solved following the first author's new technique (Das Gupta, 1978b) based on Laplace transformation and Wiener-Hopf technique, to obtain the angular neutron density at any depth. Criticality condition is derived.     

Release of protons with energy >1018 eV from intergalactic magnetic field into interstellar magnetic field

Drift instabilities arising when accelerated protons are trapped in the intergalactic medium are examined. If α, the ratio of total (plasma + energetic particles) pressure and magnetic field pressure is larger than some value α?0.1 to 0.3, the magnetic trap is destroyed and protons are released into interstellar medium. If α<α^*, the trapped protons exhibit gradient instability due to magnetic drift resonance. This ‘universal’ instability results in rapid development of strong Alfvén wave turbulence with small wavelengths transverse to the magnetic field. Particle diffusion due to the waves has a rather complicated character and appears to be weak as compared to quasilinear diffusion.     

Clustering of Earthquake Events in the Himalaya - Its Relevance to Regional Tectonic Set-up

The earthquake events of Himalaya of magnitude ≥5.0 from the time window 1905–2000 are statistically analysed. The inter-event time between earthquakes shows Hurst phenomena of temporal clustering which are spatially located in five distinct domains along the Himalayan fold-thrust belt. Out of these, two domains, one around Uttaranchal-Nepal border and the other around Nepal-Sikkim border reveal maximum number of temporal clusters and thus considered as seismically most potential zones of the Himalaya. Both these zones are located at the interface of the orthogonally disposed major tectonic discontinuities of the Peninsular Shield and Himalayan fold-thrust belt. Such zones are geologically most favourable locales for strain accumulation during later-tectonic movement. Statistical analysis points towards a probability of recurrence of seismic events in near future in these two zones. However, validity of such statistical results can be ascertained by detailed geological and geophysical modelling of the terrain.     

Contrasting parageneses in the manganese silicate-carbonate rocks from Parseoni,Sausar Group,India and their interpretation

Mn silicate-carbonate rocks at Parseoni occur as conformable lenses within metapelites and calc-silicate rocks of the Precambrian Sausar Group, India. The host rocks are estimated to have been metamorphosed at uppermost P-T conditions of 500–550°C and 3–4 kbar. The Mn-rich rocks contain appreciable Fe, reflected in the occurrence of magnetite₍₁₎ (MnO 1%), magnetite₍₂₎ (MnO 15%) and magnetite₍₃₎ (MnO 10%). Two contrasting associations of pyroxmangite, with and without tephroite, developed in the Mn silicate-carbonate rocks under isothermal-isobaric conditions. The former assemblage formed in relatively Fe-rich bulk compositions and equilibrated with a metamorphic fluid having a low X _{CO 2} (<0.2), and the latter equilibrated with a CO₂-rich fluid. Rhodochrosite+magnetite₍₁₎+quartz protoliths produced the observed mineral assemblages on metamorphism. Partitioning of major elements between coexisting phases is somewhat variable. Fe shows preference for tephroite over pyroxmangite at the ambient physical conditions of metamorphism. Oxygen fugacity during metamorphism was monitored at or near the QFM buffer in tephroite bearing domains, and the fluid composition was buffered by mineral reactions in respective domains. As compared to other metamorphosed Mn deposits of the Sausar Group, the Mn silicate-carbonate rocks at Parseoni were, therefore, metamorphosed at much lower f _{O 2} through complex mineral-fluid interactions.